Unless Perl 5, where the interpreter IS the de-facto specification, Perl 6 has the advantage of clear definition, which becomes clearer every day, since the language design team working on the syntax and the implementers finding ambiguous spots. Mainly the spec has 3 parts:

The Synopses, the (almost) human readable documents which constantly kept up to date.

The STD.pm, a Perl 6 grammar, that defines the Syntax and is part of some implementations.